In recent years, an automatic meter reading system has been introduced, in which a meter attached to a building structure such as a house measures a usage amount of gas, electricity, or tap water and measurement data is gathered by radio (wireless) communication. In the automatic meter reading system, there is a need for a small-sized radio device having a built-in antenna because of easiness of attachment to the meter box.
As an example of the above stated radio device, there is a radio device including a board-mounted planar antenna in which a grounded conductor plate and a short-circuit conductor in a radiation conductor section are connected together via a wiring pattern in a printed circuit board. In this radio device, the grounded conductor plate is used as a ground of the radiation conductor section and is provided between the radiation conductor plate and the printed circuit board (e.g., see Patent Literature 1).
In a radio communication device, an elongated conductor section is provided above a printed board such that the elongated conductor section faces the printed board. A ground pattern is placed on the printed board, and a portion of the ground pattern which faces the elongated conductor section is removed. The elongated conductor section is electrically connected to the ground pattern via a grounded conductor section and electrically connected to a power feeding point of a printed board via a power feeding conductor section. Because of this, in the vicinity of the grounded conductor section, the ground pattern is disposed between the printed board and the elongated conductor section (e.g., see Patent Literature 2).
In a radio device for automatic meter reading including the radio device, the radio device and a planar antenna are accommodated into a resin-made casing, and the radio device is disposed inside a metal-made casing. The planar antenna is connected to the metal-made casing via a metal connecting section, and the metal-made casing is used as a ground of the planar antenna. A power feeding section is provided between the metal-made casing and the planar antenna (e.g., see Patent Literature 3).
Patent Literature 1: Japanese Laid-Open Patent Application Publication No. Hei. 10-313212
Patent Literature 2: Japanese Laid-Open Patent Application Publication No. 2003-92510
Patent Literature 3: Japanese Laid-Open Patent Application Publication No. Hei. 9-27092